Active optical cables (AOCs) consist of transmitting and receiving elements which are built into the connectors and optically transmit a data signal over the fiber optic cable on the route between them. In this context, low-power laser drivers are needed for low operating voltages.
A laser driver for a data signal must, on the one hand, modulate the laser current used and, on the other hand, control the average modulation current generated thereby. The difficulty consists in achieving, on the one hand, highest switching speeds and, on the other hand, setting a specified working point of the laser.
The laser current and switching status of a laser are often set with separate circuit elements, so that two circuit elements (transistors) with a corresponding drop in voltage are required in series for this. However, the at least required drop in voltage together with the laser limits the applicability within predefined operating voltage limits.
Driving laser diodes in CMOS technologies with data signals for direct modulation is often performed in source follower configuration to take advantage of the so-called Class AB characteristic, that is, the dynamic current variation, of this configuration in the low-impedance driving of the capacitance of the laser diode.
FIG. 1 shows a schematic diagram of a simple driver circuit for a laser diode 200 used here as a charge element with a field effect transistor 201 in source follower circuit. In this case, D is a drain connection of the field effect transistor 201, G is a gate connection of the field effect transistor 201 and S is a source connection of the field effect transistor 201.
A main current path, that is, the drain-source path, of the field effect transistor 201 is arranged in series connection with the laser diode 200 between an operating voltage connection 202 at which an operating voltage VB is delivered and ground 203.
A gate-source voltage VGS is applied between the gate connection G and the source connection S, which voltage is referred to as a working point DC voltage Vt, also referred to as a threshold voltage, and a modulation AC voltage ΔV, also referred to as voltage for the modulation of the field effect transistor, is composed as its sum:VGS=Vt+ΔV 
A forward voltage VF is applied across the laser diode 200 as a load.
The total required drop in voltage Vsum across the laser diode 200 and the driving transistor, that is, the field-effect transistor 201, in particular a MOSFET, must therefore contain at least:                the forward voltage VF of the laser diode 200;        the threshold voltage Vt of the driving MOSFET 201;        the voltage ΔV for the modulation of the MOSFET 201:Vsum≥VGS+VF=Vt+ΔV+VF         
In order to be able to use this configuration in low-power applications, that is, applications for low operating voltages, either an operating voltage VB available for the driver must be at least correspondingly high, as shown schematically in the circuit arrangement according to FIG. 2, or within the circuit a higher second operating voltage VB2 must be generated as an external first operating voltage VB1, as shown schematically in FIG. 3.
In the circuit arrangement according to FIG. 2, both the series connection of the laser diode 200 and the main current path, that is, the drain-source path of the field effect transistor 201 and a laser driver, thus a driving circuit 204 for the field effect transistor 201, are arranged between the operating voltage connection 202 and ground 203. The operating voltage VB required for driving must be selected to be correspondingly high, which leads to a high power consumption of the circuit arrangement.
In the circuit arrangement according to FIG. 3, which shows an example for the internal generation of a higher operating voltage, the driving circuit 204 for the field effect transistor 201 is connected via a DC-DC converter 205 to the first operating voltage VB1, which incidentally is also supplied to the operating voltage connection 202 and is dimensioned such so that the series connection of field effect transistor 201 and laser diode 200 can be operated.
The comparatively higher second operating voltage VB2 for supplying the driving circuit 204 is taken from an output 207 of the DC-DC converter 205, to which the first operating voltage VB1 is supplied at an input 206. A high circuit complexity is required for this configuration; in addition, when using the DC-DC converter 205, a precise analysis of occurring time constants is necessary to avoid interference.
The laser current, that is, the current through the laser diode 200, is often either only controlled or measured at a series resistance, which causes an additional drop in voltage.
Laser drivers in a so-called common-source configuration are considered to be poorly suited for high data rates or fast signals due to the Miller capacitance of the transistor.